The invention relates to fluid control and actuation systems. More particularly, it pertains to the application and release of friction elements used in a power transmission for a motor vehicle.
In an automatic transmission, the various speed ratios result by selectively engaging and disengaging friction elements, the hydraulically actuated clutches and brakes. The applied and released condition of the friction elements operate to interconnect and disconnect elements of the planetary gearsets in order to produce multiple forward drive gear ratios and reverse drive. The friction elements are applied and released in response to the pressurized and vented state of a hydraulic servo through which the friction elements are actuated.
The magnitude of torque transmitted by the various friction elements in the several gear ratios is reflected in the magnitude of pressure applied to each friction element. When the magnitude of transmitted torque is high, the magnitude of actuating pressure is high. Generally, during operation in the lowest forward drive gears and reverse gear, the transmitted torque magnitude is high. Generally, a control system for an automatic transmission produces line pressure in a range up to 300 psi.
The forward clutch is engaged in each of the forward gear ratios and is disengaged in reverse drive and the park and neutral ranges, which are selected by the vehicle operator's manual control of the gear selector lever. In order to produce smoothly controlled automatic gear ratio changes in an automatic transmission, especially gearshifts between the lowest forward gear ratios and between forward drive and reverse drive, it is important to control the rate of engagement of the oncoming friction elements, including the forward clutch, that produce those gear changes.
This can be accomplished by passing hydraulic fluid through an orifice whose diameter is sized to produce a relatively slow rate of fluid flow to the forward clutch. However, it is a requirement of transmission design that an automatic transmission must be capable of rock-cycling between forward drive and reverse drive in order to free the wheels from contact with a slippery surface. This imposes the additional requirement that hydraulic fluid be rapidly vented from the forward clutch. Therefore, an orifice that produces slow flow to engage the clutch will not allow rapid flow to vent the clutch.
There is need, therefore, for a low cost, reliable technique to both fill the forward clutch at a slow flow rate and to vent fluid from the clutch rapidly.